ext4: Don't claim block from group which has corrupt bitmap

In ext4_mb_complex_scan_group, if the extent length of the newly
found extentet is greater than than the total free blocks counted
in group info, break without claiming the block.

Document different ext4_error usage, explaining the state with which we
continue if we mount with errors=continue

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Mingming Cao <cmm@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>